The combustion of solid propellants is a progressive phenomenon localized on the surface of the propellant grain. The burning rate, assuming homogeneous ignition, is defined as the distance traveled per second by the flame front perpendicularly to the exposed surface of the grain.
The burning rate is dependent upon the pressure of the surrounding gas phase. The relationship may be expressed: r = K .times. P.sup.n wherein r is the burning rate, K is a proportionally constant, P is the absolute pressure and n is the pressure exponent. It is apparent that when n is positive, increase in pressure will lead to increased burn rate and that the greater n is, the greater will be the increase in r for a given rise in P.
A propellant with a high burning rate expells a larger amount of gases in a given period of time than a slower burn rate propellant. The result is a higher mass flow rate to perform a desired function.
A catalyst is frequently used to transform a slower burning propellant into a faster burning one. A wide variety of catalytic materials are known to be useful for control of burning rate. Typical of these are materials such as iron oxide, ferrocene, copper oxide, copper chromite, various organometallic compounds, carborane and various carborane derivatives.
It is frequently advantageous to reduce the pressure exponent of a propellant so as to reduce the fluctuation in pressure caused by a change in burn rate induced, for example, by irregularity in manufacture of the propellant grain. A low pressure exponent normally is indicative of a low temperature sensitivity characteristic, and therefore has less effect on pressure with changes in temperature where the burning is conducted in a combustion chamber from which the combustion products are exhausted, as in a rocket.
While none of the above mentioned burn rate catalysts are known to have the ability to also reduce the pressure exponent, the catalyst of the instant invention possesses this property in both aluminized and non-aluminized solid composite propellants.